What causes episodic tremor and slip in subduction zones?

If tectonic plates were perfectly smooth, they would easily slide past each other. Instead, the plates are relatively rigid and jagged, so portions of the Juan de Fuca plate stick to the underside of the North American plate as it descends.

Near where the Juan de Fuca plate starts its descent, the edges of the plates lock together for hundreds of years and snap apart in great earthquakes up to magnitude 9.0. At 50 kilometers (30 miles) in depth, the temperature is so high that the plates easily slide past each other. Episodic Tremor and Slip (ETS) occurs in the area between these two sections at 25 to 40 kilometers (15 to 25 miles) below the surface.

What triggers ETS is currently a mystery that researchers are trying to solve through careful analysis of the available data. Two current hypotheses are that ETS is related to fluids that may lubricate the fault or to fluids migrating through the pore spaces in the rocks that weaken the rocks.